Download the desired theme from here. and then extract it to your Desktop or wherever you downloaded it to. (I recommend Minimal Dark) Keep this window where the file is extracted to open for now.

Open a new file explorer window. Navigate to C:\Program Files\Aseprite

If this isn’t the installation directory you used, or you’re unsure of where it is, try right clicking the Aseprite launcher icon and click open file location.

Alternatively, if using Steam, right click Aseprite, and then go to Properties, click the Local Files tab, and then click Browse Local Files.

Once in the Aseprite installation directory, open the data folder, and then open the extensions folder.

Navigate to the other window you kept open with the extracted theme (the whole folder with all the files it included). Now, drag that folder into the extensions folder within the Aseprite directory.

If you have Aseprite open at this point in time, close it and relaunch the program.

Once the program is open again, go to Edit, Preferences, and in the window that pops up, click on the Themes option on the left hand side. If you downloaded the theme “minimal dark” for example, there should now be an option to select it.

In the case of Minimal Dark, if Aseprite now looks all blurry, you need to download the other file included in its download page called minimal dark pixel font, and repeat the above process - placing it into the extensions directory - and selecting the new option for it that appears in theme selector.